Homer- John Cena. The person the whole show runs around- even if it's technically an ensemble, he's the protagonist by far- even if the whole area would do better to throw him to the wolves and leave him.
Marge- Stephanie McMahon. In small doses, she's very necessary to keep things moving along and keep the show interesting. The only problem is they never put her in small enough doses, and the show suffers as a result.
Bart- Rob Van Dam. One of the veterans who got a lot of modern fans into the whole thing and made it popular. As the show matured, he started to take a backseat to other people, and is now another part of the ensemble who does the same things that made him so beloved at the beginning.
Lisa- AJ Lee. Just interesting enough so that you can't have a season without at least one story devoted to her, but just annoying enough so that the story devoted to her is invariably going to suck.
Maggie- Kofi Kingston. Been around since the beginning, but never talks much and really hasn't done much of anything.